Equipment for use in rock or concrete machining is available in variants with percussion, rotation, and percussion with simultaneous rotation. It is well-known that the impact mechanisms that are components of such equipment are driven hydraulically. A hammer piston, mounted to move within a cylinder bore in a machine housing, is then subject to alternating pressure such that a reciprocating motion is achieved for the hammer piston in the cylinder bore. The alternating pressure is most often obtained through a separate switch-over valve, normally of sliding type and controlled by the position of the hammer piston in the cylinder bore, alternately connecting at least one of two drive chambers, formed between the hammer piston and the cylinder bore, to a line in the machine housing with driving fluid, normally hydraulic fluid, under pressure, and to a drainage line for driving fluid in the machine housing. In this way a periodically alternating pressure arises that has a periodicity corresponding to the impact frequency of the impact mechanism.
It is also known, and has been for more than 30 years, to manufacture slideless hydraulic impact mechanisms, also known sometimes as “valveless” mechanisms. Instead of having a separate switch-over valve, the hammer pistons in valveless impact mechanisms perform also the work of the switch-over valve by opening and closing the supply and drainage of driving fluid under pressure during the motion of the piston in the cylinder bore in a manner that gives an alternating pressure according to the above description in at least one of two drive chambers separated by a driving part of the hammer piston. A precondition for thus to work is that channels, arranged in the machine housing for the pressurisation and drainage of a chamber, open out into the cylinder bore such that the openings are separated in such a manner that direct short-circuited connection between the supply channel and the drainage channel does not arise at any position during the reciprocating motion of the piston. The connection between the supply channel and the drainage channel is normally present only through the gap seal that is formed between the driving part and the cylinder bore. Otherwise, major losses would arise, since the driving fluid would be allowed to pass directly from the high-pressure pump to a tank, without any useful work being carried out.
In order for the piston to continue its motion from the moment at which a channel for drainage of a drive chamber is closed until the moment at which a channel for the pressurisation of the same drive chamber opens, or vice versa, it is required that the pressure in the drive chamber change slowly as a consequence of a change in volume. This may take place through the volume of at least one drive chamber being made large relative to what is normal for traditional impact mechanisms of sliding type. It is necessary that the volume be large since the hydraulic fluid that is normally used has a low compressibility. We define the compressibility κ as the ratio between the relative change in volume and the change in pressure: κ=(dV/V)/dP. It is, however, more common to use the modulus of compressibility, β, as a measure of compressibility. This is the inverse of the compressibility as defined above, i.e. β=dP/(dV/V). The units of the modulus of compressibility are Pascal. The definitions given above will be used throughout this document.
U.S. Pat. No. 4,282,937 reveals a valveless hydraulic impact mechanism with two drive chambers, where the pressure alternates in both of these chambers. Both drive chambers have a large effective volume through them being placed in permanent connection with volumes that lie close to the cylinder bore. One disadvantage of the prior art technology revealed in this way is that it has turned out to give a surprisingly low efficiency, given that one mobile part has been removed compared with conventional impact mechanisms with a switch-over valve. In this document we define “efficiency”, unless otherwise stated, as the hydraulic efficiency, i.e. the impact power of the piston divided by the power supplied to the hydraulic pump.
SU 1068591 A reveals a valveless hydraulic impact mechanism according to a second principle, namely that of alternating pressure in the upper drive chamber and a constant pressure in the lower, i.e. the chamber that is closest to the connection of the tool. What is aspired to here is improved efficiency through the introduction of a non-linear accumulator system working directly against the chamber in which the pressure alternates. This is shown with two separate gas accumulators, where one of these has a high charging pressure and the other has a low charging pressure.
One disadvantage of being compelled to introduce accumulators that act directly at a chamber where the pressure alternates at the impact frequency between full impact mechanism pressure and a low return pressure during operation is that the service interval becomes shorter due to the moving parts in the accumulators being subject to heavy wear.
Purpose of the Invention and its Most Important Distinguishing Features
One purpose of the present invention is to demonstrate a design of a valveless hydraulic impact mechanism that offers the opportunity of improving the efficiency without at the same time reducing the service interval. This is achieved in the manner that is described in the independent claims. Further advantageous embodiments are described in the non-independent claims.
We define the effective volume of the drive chambers as the sum of the drive chamber volumes that have an alternating pressure during one stroke cycle, including volumes that are in continuous connection with one and the same drive chamber during a complete stroke cycle. It has proved to be the case that the effective volume of the drive chambers, according to the definition given above, is of crucial significance for the efficiency of the impact mechanism with respect to valveless impact mechanisms. There are, of course, many factors that influence the efficiency, such as play and the length of gap seals, friction in bearings, etc. It is not possible, however, to achieve the desired efficiency without a correctly adapted effective volume of the drive chambers, no matter how such play and bearings are designed.
Factors that influence the optimal effective volume of the drive chambers with respect to efficiency are: the impact mechanism pressure used, the compressibility of the driving medium and the energy of the piston in its impact against the tool or against a part that interacts with the tool. To be more precise, the effective volume of the drive chambers is influenced in inverse proportion to the square of the impact mechanism pressure and proportionally to the product of the effective modulus of compressibility of the driving medium and the energy of the hammer piston when it impacts the tool or a part that interacts with the tool, such as the part known as an “adapter”.
The relationship can be expressed by the equation: V=k*β*E/p2, where V is the effective drive chamber volume (by which we mean the sum of the volumes of the two drive chambers, including volumes that are in continuous connection with one and the same drive chamber during a complete stroke cycle). In the case in which alternating pressure is present in only one of the drive chambers, the volume of this chamber is normally totally dominating in comparison with that of the chamber that has a constant pressure. It then becomes possible to regard the effective drive chamber volume as the volume solely of the drive chamber that has alternating pressure together with the volume that is continuously connected to this. β in the equation constitutes the effective modulus of compressibility of the driving medium as it has been previously defined. If the driving medium consists of several components each of them having an individual compressibility, the effective modulus of compressibility is calculated as the resultant ratio between the change in pressure and the relative change in volume. FIG. 3 presents values of β for hydraulic fluids with different levels of air content. FIG. 3 has been taken from a collection of equations in hydraulic and pneumatic engineering, and thus constitutes prior art technology. It will be apparent to one skilled in the arts that β=1500+7.5p MPa when the air content of the fluid is zero. In the case in which gas accumulators are directly connected to the effective volumes, as is described in, for example, SU 1068591 A, these are also to be included in the calculation of effective volume. Thus, the existing gas volume that is present in these, normally consisting of nitrogen gas, will be included in the calculation of the effective modulus of compressibility. It is appropriate in this case that the gas volumes of the accumulators when the impact mechanism is in its resting condition, i.e. the condition that normally prevails before the impact mechanism is started, be used. The said gas accumulators here are not to be confused with those that are normally connected to the supply line and return line for the impact mechanism. Such accumulators are connected to the drive chamber only intermittently, and are thus not to be included in the calculation of the effective volume or the effective modulus of compressibility.
Furthermore, E denotes the impact energy of the piston in its impact with the tool or with a part that interacts with the tool. Finally, p is the impact mechanism pressure that is used. The impact mechanism pressure is normally between 150 and 250 bar. Finally, k is a constant of proportionality, that it has become apparent most suitably lies in the interval 7.0≦k≦9.5, but where a good effect for the efficiency can be achieved in the larger interval 6.2<k<11.0 and even up to the interval 5.3-21.0.
When the volumes have been dimensioned according to the description above, it is possible to achieve an efficiency that exceeds 75% in the case in which the effective drive chamber volumes are limited by walls of non-flexible material, i.e. when the driving medium consists of pure fluid or fluid that has been mixed to a certain extent with gas while, in contrast, no gas accumulators are continuously directly connected to the drive chambers. It is possible to achieve such efficiencies without requiring extremely low play between the piston and the cylinder bore, and thus without the subsequent extremely high demands on manufacturing precision needing to be used. An appropriate play may be 0.05 millimeter. This form of impact mechanism is that which gives the longest service interval of all, since so few moving parts are included.
Very much smaller effective drive chamber volumes can be achieved if gas accumulators are continuously connected to the drive chambers and in this way are included in the calculation of effective volumes, as previously described. Furthermore, even higher efficiencies can be achieved in the impact mechanism if two gas accumulators with different specifications are connected to one and the same drive chamber in such a manner that one is pre-charged with a high gas pressure, i.e. equal to the impact mechanism pressure or the system pressure, and one is pre-charged with a low gas pressure, normally atmospheric pressure. When the dimensioning of volumes takes place as described earlier, an efficiency that exceeds 85% can be achieved with a play of the same magnitude as that previously mentioned. The service interval is increased also in this case, through the volumes not being made larger than necessary. The need for motion of the membrane of the accumulators can in this way be reduced.
One preferred embodiment constitutes an impact mechanism, where the volume (by which we refer to the effective volume as defined above) of one of the drive chambers is much larger than that of the second drive chamber, i.e. that the volume of the second drive chamber is negligible, for example 20% or less than the volume of the first drive chamber, and where the smaller drive chamber has essentially constant pressure during the complete stroke cycle. Constant pressure in this chamber is normally achieved by the chamber being connected to a source of constant pressure during the complete stroke cycle, or at least during essentially the complete stroke cycle, most often being directly connected to the source for the system pressure or alternatively impact mechanism pressure.
Impact mechanisms of the type that has been described above can be an integrated component of equipment for the machining of at least one of rock and concrete, such as rock drills and hydraulic breakers. These machines or breakers during operation should most often be mounted onto a carrier that can comprise means for their alignment and position together with means for the feed of the drill or breaker against the rock or concrete element that is to be machined, and further, means for the control and monitoring of the process. Such a carrier may be a rock drilling rig.